Osteolytic bone disease is a major contributor to the morbidity and mortality caused by incurable multiple myeloma (MM). Active 1,25-dihydroxyvitamin D (1,25-(OH)2D) stimulates both tumor & bone cells, contributing to myeloma bone disease (MBD), despite vitamin D insufficiency in the majority of MM patients. This continuing function of vitamin D signaling in MBD is due to hypersensitivity of specific gene promoters in both tumor cells & bone cells to unusually low concentrations (10-11-10-10M) of 1,25-(OH)2D. Extensive preliminary data support a novel mechanism of action of 1,25-(OH)2D in MBD: Both tumor cells and bone cells are hypersensitized to 1,25-(OH)2D by overexpression of the transcriptional activating factor TAF12. At very low levels of expression in normal cells, TAF12 functions as a widely expressed, conventional transcription factor complex component, but when overexpressed it binds at vitamin D receptor (VDR)- responsive promoters and functions as a novel VDR co-activator. TAF12 was strongly overexpressed in 5/8 primary MM samples, while TAF12 gain and loss of function respectively increased and decreased sensitivity of specific transcription to 1,25-(OH)2D. Additional vitamin D hypersensitivity may be due to binding of ATF7 (also overexpressed in MM) to TAF12 & frequent loss of the VDR co-repressor SMRT (NCoR2) from MM cells. AIM 1 will investigate the biological responses of myeloma & marrow stromal cells to low concentrations of 1,25-(OH)2D by assaying MM cell proliferation, MM-stromal cell adhesion, cytokine production & support of osteoclast formation. We will test whether ATF7 is consistently overexpressed in primary MM cells and if it occurs in concert with TAF12. The mechanism by which TAF12 expression is increased will also be investigated and may be caused by increased IL6 & TNF in the marrow microenvironment. AIM 2 will characterize the molecular mechanisms of vitamin D hypersensitivity in tumor & bone stromal cells, including assembly of TAF12 & TAF12:ATF7 complexes on two vitamin D-responsive promoters, assayed by chromatin immunoprecipitation The effects of TAF12 & TAF12:ATF7 on VDR protein half-life in response to 1,25-(OH)2D will be determined in MM and bone stromal cells, as will the ability of the co-repressor SMRT to reverse hypersensitivity to 1,25-(OH)2D. In Year 2, AIM 3 will test TAF12 as a therapeutic target in myeloma. MM cells with TAF12 gain or loss of function will be grown in an immunocompromised mouse model of osteolytic MBD to determine the effects on tumor burden and bone destruction. The goal is to demonstrate the utility of TAF12 as an important regulator of MBD, since co-activator:VDR binding interactions are druggable with small molecules. Such novel agents could selectively and effectively target the pathological responses to 1,25-(OH)2D of tumor cells and bone cells in myeloma bone disease, without the systemic toxicities caused by vitamin D antagonists.